A subsea cable, also called a submarine or undersea cable, is a fiber-optic cable laid along the seabed to carry data between countries and continents. Modern systems hold a number of fiber pairs inside protective layers, with powered repeaters along the route that boost the signal over long distances. Subsea cables carry the large majority of intercontinental internet and private network traffic, so they sit behind most international circuits a business buys, whether or not the contract names them.
At a glance
- Subsea cables carry most data traffic between continents; satellites carry a much smaller share.
- Each cable lands at a coastal landing station, where it connects to land networks.
- Systems are often owned by consortiums of carriers, by individual carriers, or by large cloud and content companies.
- Faults from anchors, fishing and natural events happen regularly, and repairs can take days to weeks.
- Diversity and latency depend on which cable systems and landing stations a specific circuit uses.
What problem it solves
Organizations with offices, customers or data centers in more than one region need to move data across oceans. Satellite links can do it, but with less capacity and, for traditional geostationary satellites, much higher latency. Subsea fiber offers very large capacity and comparatively low latency over intercontinental distances.
For buyers, the cables themselves are usually invisible: you buy an international private line, a wavelength, or a global WAN service, and the provider routes it over one or more cable systems. Understanding the cables matters when performance, resilience and cost depend on which route your traffic actually takes.
How it works
The cable. A subsea cable holds a number of fiber pairs at its core, surrounded by layers that protect it and carry power. Near shore, where risk from anchors and fishing is higher, cables are typically armored and often buried.
Repeaters and line systems. Repeaters placed along the route amplify the light signals. Terminal equipment at each end uses dense wavelength division multiplexing (DWDM) to carry many high-capacity channels on each fiber pair.
Landing stations. Each end comes ashore at a cable landing station, which connects to terrestrial backbone and middle-mile networks running to data centers and cities. Several cables may share a landing station or nearby land routes, which can create shared points of failure.
Ownership and capacity. Cables are built and owned by consortiums of carriers, by individual carriers, or increasingly by large cloud and content providers. Capacity is sold or leased as fiber pairs, spectrum, wavelengths or smaller circuits, and resold many times over.
Faults and repair. When a cable is damaged, traffic on affected circuits may be rerouted onto other cables if the provider has capacity and protection in place. Physical repair needs a cable ship, and timing depends on location, weather, permits and ship availability.
To plan international connectivity across these routes, see our Global WAN Services solution page.
When it matters for buyers
- Expanding internationally. Opening offices or serving customers in another region means your traffic will cross one or more cables.
- Designing international redundancy. Two circuits are only as independent as their cable systems, landing stations and land routes; path diversity has to be verified route by route.
- Latency-sensitive workloads. Voice, video, trading and database replication between regions depend on the specific route; a longer path can add noticeable delay.
- Choosing a global network provider. Providers differ in which cable systems they use and whether they own capacity or lease it.
- After a regional outage. A cable cut is a prompt to check whether your backup route shares the same system.
Questions to ask vendors
- Which cable systems and landing stations will our primary and backup circuits use?
- Will you state in writing that the two circuits use different cable systems, landing stations and land routes?
- Is this circuit protected, and if a cable fails, what is the restoration path and its expected latency?
- What round-trip latency do you commit to on this route, and how is it measured?
- Do you own capacity on these cables, or lease it from another carrier?
- What has your typical restoration time been when a cable on this route has failed?
How it differs from an international private backbone
A subsea cable is physical infrastructure: fiber on the seabed between landing stations. An international private backbone is a service built by a provider across many cables and land networks, giving customers private connectivity between regions under one contract and SLA. The backbone provider decides which cables to use and how to protect your traffic. When you compare backbones, the underlying cable systems, and how much diversity the provider actually maintains between them, are among the things worth asking about.